In addition to how the milk is pasteurized, and whether you touch the opening, and whether you leave or on the counter, it depends on how cold your fridge is. Mine is barely above freezing (sometimes slightly below freezing) and I’m constantly surprised at how long our milk keeps.
Also, the ultra pasteurized stuff keeps a lot longer after opening it than the regular milk does, presumably because we don’t generally contaminate either, and the UP milk starts with less stuff that can grow.
Yep, old milk makes biscuits or the sweet cornbread the kids love so much.
Around here we’re more apt to be out of milk than have “old” milk, tho’.
Buttermilk will last months if kept cold. It’s a dangerous thing if left out. Explosive buttermilk is not pleasant. You gotta know you were bringing it up to your face at opening, to smell(why? It’s buttermilk, not a good odor, but it’s ingrained, open the carton and smell). Makes your skin so soft, tho’
Nobody seems to have mentioned Filtered Milk, which is a thing in the UK for semi-skimmed at least. It is put through filters fine enough to strain out bacteria. Costs more but lasts at least twice as long as standard semi-skimmed milk.
Huh, is that supposed to be the benefit? They have it here, too. But it feels like a way of turning a fairly natural food into a highly processed one, and i wonder what else gets filtered, and if proteins get mangled into different proteins.
There’s also irradiated milk which comes in juice box type cartons and is not refrigerated, at least not on the shelf. It’s not really sold in the US because hurr durr radiation scary, but I remember seeing it in Jamaica and it had a six month shelf life. Not sure how long it would last after being opened, but…
I buy Kroger brand organic milk because it’s the only one they sell that’s ultra pasteurized, and I don’t go through much milk, but when I need it I need it. The sell-by date is usually six weeks from the time it’s put out for sale. The half gallon carton I have right now is dated today, and I know I opened it at least a month ago. It’s only just starting to get a slight hint of going off. Fortunately this milk only goes bad gradually, so it’s not a case of “oh damn this milk went bad since yesterday” but “hmm, I should probably finish this up within a week.” I have had one or two go bad way early, like weeks before the expiration date, probably due to my own contamination. So the type of pasteurization does have some effect on longevity even after the container is opened. Even irradiated milk could be limited to one week in that case, but it can also last a lot longer if open time is minimized.
I have a similar relationship with sour cream. I’ll buy the tiniest 8oz tub they sell, and I generally only use half of it by the time I decide to replace it. Often that’s a month or six weeks after the expiration date. I think those have a similar timeframe as the ultra pasteurized milk, or maybe four weeks instead of six. Still, I can usually get double the amount of time. Once though I opened a container that had gone moldy after only two weeks. I noticed a dribble of cream had fallen over the rim and down the outside. That provided a pathway for mold that grew on the glob sitting in the open in the refrigerator to get up under the lid and inside the tub. Live and learn.
It’s still milk. Filtering isn’t going to cause any changes in protein.
Actually, there is a brand in the United States called Fairlife milk (a subsidiary of Coca-Cola) that advertises its milk has 50% more protein and 50% less sugar, so they are using the filtration process to increase the protein content.
Not that this answers the question, but sour milk is very popular in my home country, Zimbabwe.
I think is is linked to the gene for being able to digest fresh milk. I’m fine with both, but I come from a European heritage.
Sour milk, colloquially “amasi” is typically sold in all groceries.
I use it to bake bread, it adds a distinctive flavour.
Whipping egg whites, just mechanically beating them, denatures a lot of the proteins.
I agree with Beck on this. Buttermilk will last quite a long time in the fridge.
My experience with fresh milk stored in the fridge is that it turns sour very suddenly, very close to the use-by date. It remains stable for maybe 9 days and then suddenly, within a matter of hours, it is completely undrinkable. Not only are the smell and taste unmistakeable, but if you put it in coffee or tea it will curdle.
I don’t share the experience reported above of milk lasting well beyond the use-by date. I suspect this is a difference in processing in different markets.
Buttermilk is a different matter entirely. I routinely use buttermilk that has been in the fridge for weeks after the use-by date. And yes, like Beck I always smell it when I open the container. I actually like the smell of buttermilk - it’s sour yes, but in a wholesome way.
I’m not sure of what you’re getting at. Cooking eggs denatures the tertiary structure of the proteins, but you still eat them for the protein (among other things).
My daughter left an opened quart bottle of ultra-pasteurized Darigold half & half in my fridge, half full. It was opened at least 3 months ago. It’s still good. It’s marked “best by 10/26/25”.
I just tossed it because we got a new bottle for Thanksgiving pies and I’d never serve anyone something that old.
I do. I eat meringue, too. But cooked eggs and whipped eggs have been eaten for centuries and are generally known to be safe. The ultra-filtered milk is probably safe, too. But it’s not “just the same as regular milk”, any more than pasteurized/cooked milk (also with centuries of safety data) is exactly the same as raw milk. Maybe I’m being totally irrational, but the idea creeps me out.
Maybe I’m misunderstanding, but you’re not trying to drink raw milk, you know the pasteurization denatures the proteins, so isn’t filtering (if it indeed denatures more proteins) just a higher degree of denaturing/“cooking”?
It sounds akin to felting wool in a washing machine, where the agitation alone causes the fibers to tighten up, even without added heat. 
I drink pasteurized milk. I don’t drink ultra-filtered milk. I doubt they are the same. The ultra-filtered milk is probably totally fine. But it kind of creeps me out.
I’ve had the same experience, and in fact was going to mention that here.
(I also found a local source for raw milk, labeled “Not for human consumption” but of course that’s what I’m going to do with it, and its expiration date is 4 days from now.